The present invention relates to a method and apparatus for coding heat-sensitive optically-readable media. The invention is particularly applicable to optically-readable value cards formed with a plurality of windows each representing a value element for use in vending systems, such as described in our U.S. Pat. No. 4,505,498.
As described in the above-cited patent specification, there are many consumer goods and services which are conveniently purchased directly by the consumer using coins or special tokens. Examples of such services are telephone calls, parking space for vehicles, subway or bus fares, locker rentals, and in some countries, the gas utility; examples of goods are vending machine articles, such as newspapers, food products and the like. However, the ordinary token-operated or coin-operated systems suffer from several disadvantages, including the labor and infra-structure required for collecting, counting and recycling the tokens or coins, and the danger of vandalism and theft.
For this reason, a number of forms of value cards have been devised, which value cards include heat-sensitive optically-readable elements each representing a unit of value for the respective goods or services being vended. The optically-readable value element is usually in the form of a thermographic deposit which is heated to effect the cancellation of the element after the respective item has been vended. Each window in such a card includes a heat-sensitive material whose transparency changes upon heating. These changes in transparency may be either reversible (thermochromic), or irreversible (thermographic). The above-cited patent specification discloses such a value card wherein each window is comprised of a material which when heated to a particular temperature above ambient, undergoes a first change which is reversible, and when heated to a higher temperature, undergoes a second transparency change which is irreversible. Thus, the reversible change may be used to check the validity of the value element, and the second change may be used to effect the cancellation of the value element.
A number of heat-sensitive materials, both thermographic and thermochromic, are known. Some of these materials become more transparent upon heating, and others become more opaque upon heating.
An object of the present invention is to provide a method and apparatus for coding heat-sensitive optically-readable media to permit the media to be used only for a specified application, and not to be used for other applications.